Automatic insulation tester for grounded and ungrounded electrical equipment

ABSTRACT

An automatic insulation tester tests the insulation resistance of electrical equipment when the electrical equipment is inoperative. The automatic insulation tester is automatically rendered operative when the electrical equipment is inoperative, and is automatically rendered inoperative when the electrical equipment is operative. The automatic insulation tester applies a voltage on the order of the rated voltage of the electrical equipment to the electrical equipment and measures and displays the insulation resistance of the electrical equipment. When the insulation resistance falls below a predetermined value, an indicator is activated, and remains in an activated state until the insulation resistance of the electrical equipment rises above the predetermined value and the automatic insulation tester is manually reset. In preferred embodiments of the present invention, the automatic insulation tester is provided with a ground interrupter to allow the automatic insulation tester to be used with permanently grounded electrical equipment. A single comparator can be used to monitor several motors or generators by multiplexing the inputs to the comparator and demultiplexing the output from the comparator. The selection means for the motor or generator can be automatic, so that a periodic cycling is performed, or can be manual.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of co-pending U.S. patent applicationSer. No. 06/942,831, filed Dec. 17, 1986, which is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to insulation testers, and morespecifically to insulation testers for electrical equipment.

2. General Background

Motors and generators are used in many industries and institutions.Offshore drilling rigs typically have one or more generators to meettheir power requirements, as well as motors to perform various tasks onthe rigs. Hospitals typically have backup generators for use in theevent of a power outage.

A common cause of malfunctioning of electrical and electronic equipmentis inadequate electrical insulation of the wiring, which can be due, forexample, to manufacturing defects of the wiring, shorts introducedduring manufacture of the equipment, or breakdown of the insulation.

There exist reliable methods of and devices for testing the soundness ofelectrical insulation of insulated wires prior to use in wiring systemsand electrical and electronic equipment, both during and immediatelyafter manufacture of the insulated wires. Examples of such devices andmethods are disclosed in U.S. Pat. Nos. 3,413,541; 3,546,581; 3,789,294;3,789,295; 3,823,370; 4,313,085; and 4,160,947.

There are also numerous means for testing electrical equipment to detectany problems with electrical insulation that may have been introducedduring manufacture of the equipment. U.S. Pat. No. 4,152,640 discloses amethod of testing the integrity of insulation of windings in newlymanufactured electrical equipment by applying a positive DC voltage toboth ends of one winding, a negative DC voltage of the same magnitude toboth ends of another winding, grounding all other windings, andmeasuring the current flowing in series in the two windings beingtested. The process is repeated until each winding has been tested witha positive and a negative voltage. A similar method is disclosed inwhich AC voltages, which are of the same magnitude but out of phase, areapplied to the two windings being tested.

Even if the characteristics of the electrical insulation of wiringpasses muster during testing prior to incorporation into equipment, andduring testing before the equipment is put into use, there is noguarantee that the insulation will hold up indefinitely. Typicalsafeguards against damage to equipment due to insulation breakdown whichoccurs when the equipment is operative include fuses and circuitbreakers which limit the amount of current passing through theequipment. Some non-grounded AC equipment is provided with "grounddetectors" which constantly and automatically check the quality of theinsulation of wiring by monitoring leakage current between the wiringand ground while the equipment is running, but which are inoperativewhen the equipment is not in use. Other devices to check insulation whenequipment is running include a device disclosed in U.S. Pat. No.4,214,311 which constantly monitors insulation resistance values innon-grounded DC equipment while the equipment is in operation, and onedisclosed in U.S. Pat. No. 4,394,615 which monitors insulation betweenwiring and the housing of equipment when the equipment is on.

All too often, the potential for failure of electrical equipment causedby inadequate insulation occurs when the equipment is idle, due toinsulation breakdown. Some common causes of insulation breakdown, inaddition to thermal aging, include the presence of moisture, oil,grease, chemical fumes, and airborne contaminants. If the equipment isnot tested until it is running, the testing may be useless because, ifthe insulation breakdown is serious enough, the equipment may fail andeven explode on starting.

It is common practice today to occasionally check the insulation ofmotors and generators when the equipment is not running. A common way todo this is to open the motor or generator, find an exposed wire andcheck the resistance of the motor or generator by applying a DC voltageto the wire, and measuring the current which flows to ground (thechassis of the motor or generator). In the case of a permanentlygrounded system, the ground wire must be disconnected before the DCvoltage is applied to a winding or a wire. Whether checking a groundedor ungrounded system, this method of insulation monitoring is timeconsuming and, therefore, is not performed as often as it should be. Theinevitable result is that insulation breakdown goes undetected, and isoften not discovered until a motor or generator fails when it isstarted, at great expense to its owner. Various solutions have beenproposed to do away with the need for manually checking the insulationcharacteristics of motors when not in use. For example, U.S. Pat. No.3,611,036 discloses a ground detecting circuit for a motor wherein twoterminals of the motor are connected to two input power lines throughdiodes in series with neon lamps and current-limiting resistors. Aground (electrical contact between the wiring or windings and thechassis) in the motor produces a current flow through one or both of thelamps. A photosensitive resistor, in response to light from the lamps,activates a relay control circuit to open the input line to the motor sothat the motor cannot be turned on.

U.S. Pat. No. 3,656,136 discloses a sensing device which is connected toone of the winding terminals of the motor when the motor is notoperative. A voltage is applied to the winding terminal of the motor,and the current flowing through the terminal is monitored. When it risesabove a predetermined level, a control signal is generated, activating arelay which causes contacts between the winding terminals and the inputpower lines to open, thereby preventing the motor from being turned on.

U.S. Pat. No. 4,319,297 discloses a protective system for motors inwhich a sensing means is coupled to a motor winding to detect and signalwhen the winding resistance drops below a predetermined level. Means areprovided to decouple the motor winding from the sensing means when themotor is turned on.

While the solutions proposed in these three last-mentioned patents maybe useful for detecting when the resistance of the windings of motorsreaches an unacceptable level, they do not test the insulation, and onlyprovide an indication of whether the resistance is above or below acertain, predetermined value. Also, they cannot be used with groundedmotors or generators.

SUMMARY OF THE INVENTION

The present invention provides an automatic insulation monitor/testerwhich constantly monitors the insulation resistance of the windings ofelectrical equipment, such as motors or generators, when the electricalequipment is not in use. The monitor/tester preferably includes a highvoltage power supply which applies a high voltage, preferably greaterthan or equal to the rated voltage of the electrical equipment, to awinding of the electrical equipment, and a sensing means measures theleakage current to ground. The value of the resistance of the insulationis preferably visually displayed, allowing an operator to note when theinsulation resistance is dangerously low. When the resistance dropsbelow a predetermined value, the insulation tester produces a signal.The signal may trigger, for example, a visual alarm, an audio alarm, ora relay switch which prevents the motor or generator being monitoredfrom being started.

In preferred embodiments of the present invention, the signal actuates alatching relay which changes the state of relay contacts to open acircuit in a motor starter to prevent the motor or generator from beingstarted, disconnect input power from the sensing means, disconnect inputpower from the high voltage power supply, and provide a visualindication that the insulation resistance has dropped below thepredetermined value. The use of a latching relay is advantageous in thatthe relay contacts remain in the changed state until a reset button ismanually pushed to return the contacts to the pre-fault state. Thus,even if, when he presses the reset button returning the insulationtester to an active condition, the insulation tester indicates that theinsulation resistance has returned to an acceptable level, the operatorof the machinery can tell that it had dropped to below an unacceptablelevel, and there can be an investigation to determine the cause of thedrop in order to prevent it from happening again. For example, theinvestigation may reveal water in the motor or generator, indicatingthat it had gotten wet, causing the insulation resistance to drop.Precautions can then be made to prevent it from getting wet in thefuture.

Means are preferably provided to automatically render the insulationtester inoperative when the electrical equipment is started, and toautomatically render the insulation tester operative when the electricalequipment is inoperative.

The insulation tester preferably comprises a ground interrupter to allowit to be used with grounded electrical equipment. The ground interruptercomprises a contact, in an electrical connection between the chassis andthe windings of the electrical equipment, which is open when theinsulation tester is operative, and closed when the electrical equipmentis operative.

In some embodiments disclosed herein, a single tester can be used toautomatically test several motors or generators. The automaticinsulation monitor for multiple motors/generators comprises power supplymeans, switching means for electrically connecting the first outputterminal of the power supply means to a winding of a motor/generatorselected for monitoring, means for electrically connecting the secondoutput terminal of the power supply means to the chassis of each of themotors/generators, sensing means for determining insulation resistanceof the motor/generator selected for monitoring by measuring currentflowing between the first and second output terminals of the powersupply means, indicating means for indicating the value of insulationresistance of the motor/generator selected for monitoring, and controlmeans for automatically causing the automatic insulation monitor torefrain from monitoring a motor/generator when the motor/generator isoperative. The control means preferably comprises means to cause theswitching means to skip over operative motors/generators. The automaticinsulation monitor for multiple motors/generators preferably normallyautomatically monitors all non-operative motors/generators connectedthereto, sequentially monitoring each non-operative motor/generator fora predetermined time interval, but the switching means can preferably bemanually manipulated such that the automatic insulation monitor monitorsa desired non-operative motor/generator connected thereto at the will ofan operator. To allow it to be used with grounded electrical equipment,the automatic insulation monitor for multiple motors/generatorspreferably comprises ground-interrupting means for automaticallyinterrupting the electrical connection between the winding and thechassis, when the motor/generator is selected for monitoring, of amotor/generator normally having an electrical connection between thewinding and the chassis. The automatic insulation monitor for multiplemotors/generators may advantageously include any other featuresdisclosed in other embodiments which are compatible therewith andenhance its operation.

It is an object of the present invention to provide an automaticinsulation tester for electrical equipment.

It is also an object of the present invention to provide an automaticinsulation tester which displays the value of the insulation resistanceof the windings.

Another object of the present invention is to provide an automaticinsulation tester which produces a signal when the value of theinsulation resistance of the windings drops below a predetermined value.

It is a further object of the present invention to provide an automaticinsulation tester which prevents electrical equipment from starting whenthe insulation resistance of the windings drops below a predeterminedvalue.

It is a further object of the present invention to provide an automaticinsulation tester which can be used with grounded electrical equipment.

It is a further object of the present invention to provide an automaticinsulation tester/monitor which can be used with multiplemotors/generators, whether normally grounded or ungrounded.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be had to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like parts are given like reference numerals, and wherein:

FIG. 1 is a schematic diagram of a first embodiment of the automaticinsulation tester of the present invention.

FIG. 2 is a schematic diagram showing a typical installation of theinsulation tester shown in FIG. 1.

FIG. 3 shows an embodiment of the automatic insulation tester of thepresent invention which can be used with grounded electrical equipment.

FIG. 4 shows a typical connection of the embodiment shown in FIG. 3 togrounded electrical equipment.

FIG. 5 shows the preferred embodiment of the present invention, whichcan be used with grounded electrical equipment.

FIG. 6 shows a typical connection between the embodiment of theinvention shown in FIG. 5 and grounded electrical equipment.

FIG. 7 is a schematic diagram of a low voltage power supply inaccordance with the present invention.

FIG. 8 is a schematic diagram of a high voltage power supply inaccordance with the present invention.

FIG. 9 shows an embodiment of an instrument panel of the automaticinsulation tester of the present invention.

FIG. 10 is a schematic diagram of an embodiment of the present inventionwhich can be used to automatically test several motors.

FIG. 11 is a front elevational view of an instrument panel used inconjunction with the embodiment of FIG. 10.

FIG. 12 is a schematic diagram of portions of the switching unit of FIG.10.

FIG. 13 is a schematic diagram of portions of the power supply of FIG.10.

FIG. 14 is a schematic diagram of an embodiment of the present inventionwhich can be used to automatically test several generators.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, the circuitry of the preferred embodimentof the insulation tester of the present invention is shown in FIG. 1,and may be permanently connected to electrical equipment as shown inFIG. 2.

The electrical equipment shown in FIG. 2 comprises a three phase motor100 having a chassis 110 and windings 114, 115 and 116. Windingterminals 112 and 113 are electrically connected through lines 131, 132and 133 and a circuit breaker 140, to buss lines 101, 102 and 103,respectively. Normally-open motor relay contacts 121, 122 and 123 arepresent in lines 131, 132 and 133, respectively, and are controlled bymotor relay coil 120, which itself is activated by a starter switch 125.

Automatic insulation tester 400 comprises a low voltage power supply 20,a high voltage power supply 30, a comparator circuit 40 and a visualdisplay of resistance 50.

Input terminals 21 and 22 of low voltage power supply 20 areelectrically connected to terminal connections 1 and 2, which areelectrically connected to buss lines 101 and 102, respectively. Outputterminals 23 and 24 are electrically connected to input terminals 31 and32, respectively, of high voltage power supply 30. Output terminal 33 ofhigh voltage power supply 30 is electrically connected to terminalconnection 3, which is electrically connected to chassis 110 of motor100. A fuse 35 is electrically connected in series between test inputterminal 44 and terminal connection 4. Output terminal 34 iselectrically connected to test input terminal 43 of comparator circuit40, and test input terminal 44 is electrically connected to terminalconnection 4, which is itself electrically connected to winding terminal113 of motor 100.

Power input terminals 41 and 42 of comparator circuit 40 areelectrically connected to terminal connections 1 and 2, respectively.Visual display of resistance 50 and a control relay coil 60 are eachelectrically connected across comparator circuit 40. A normally-closedtest contact 45 is connected in series between test input terminal 44and terminal connection 4. A normally-open test contact 46 and aresistor 47 are electrically connected in series between test inputterminal 44 and output terminal 33.

A control relay coil 70 is electrically connected to terminalconnections and 12 which are electrically connected to lines 132 and131, respectively, of motor 100. A latching relay coil 80 and a releaserelay coil 90 are each electrically connected across terminal connectors1 and 2. A normally-open reset switch 91 is connected in series withrelease relay coil 90. Light emitting diodes (LEDs) 92 and 93 are eachelectrically connected across output terminals 23 and 24 of low voltagepower supply 20. Resistors R3 and R4 are electrically connected inseries with LEDs 92 and 93, respectively.

Normally-open contact 61 is controlled by time delay relay coil 60.Normally-closed contacts 71, 72 and 73 are controlled by control relaycoil 70. Normally-open contacts 82, 85 and 87 and normally closedcontacts 81, 83, 84 and 86 are controlled by latching relay coil 80 andrelease relay coil 90. Coil 80, when activated, serves to switch thestate of contacts 81-87 from that shown in FIG. 1, and coil 90 serves toswitch the state of contacts 81-87 back to that shown in FIG. 1.Contacts 81-87 are latching contacts--that is, a momentary impulse ofcurrent through latching relay coil 80 changes their state from thatshown in FIG. 1, and contacts 81-87 stay in that state until a momentaryimpulse of current through release relay coil 90 changes their stateback to that shown in FIG. 1. Contacts 61 and 71-73 are normal contacts,whose state remains changed only as long as current flows through therespective coil controlling them.

In operation, with motor 100 inoperative, automatic insulation tester400 works in the following manner. Low voltage power supply 20 receivespower at its input terminals 21 and 22 from buss lines 101 and 102, andproduces a stable low voltage at its output terminals which powers LED93 and high voltage power supply 30 through normally closed contacts 83and 72. LED 93 indicates that insulation tester 400 is operational.

High voltage power supply 30 produces a stable high voltage on the orderof, and preferably greater than or equal to, the rated voltage of motor100, across its output terminals 33 and 34.

Comparator circuit 40 measures the resistance of the insulation ofwindings 114, 115 and 116 by measuring the current flowing from testinput terminal 43 to test input terminal 44, which is equal to thecurrent flowing from windings 114, 115 and 116 to chassis 110 of motor100. This value of resistance is displayed in visual display 50 (alsoshown in FIG. 9). By viewing display 50, an operator can readily tellwhether the resistance of the insulation is sufficiently high, or isgetting dangerously low. He can take readings from display 50periodically to note any trend showing gradual degradation of theinsulation, and take preventative measures to reinsulate the windingsbefore motor 100 malfunctions due to low insulation resistance. Also, toinsure that the reading displayed is accurate, he may press test switch48 (FIG. 9) which opens test contact 45 and voltage power supply 30across test resistor 47. If high voltage power supply 30 is producingvoltage of the proper value, needle 51 (FIG. 9) of display 50 will moveto calibration set point 52. If not, the output of high voltage powersupply 30 can be adjusted, as will be explained further in thedescription of FIG. 8.

Should the value of the insulation resistance drop below a predeterminedvalue, time delay relay coil 60 energizes, changing the state of contact61 from opened to closed. Current then flows through latching relay coil80, changing the state of contacts 81-87 from that shown in FIG. 1.

Contact 81 opens, interrupting power to comparator circuit 40. Contact82 closes, allowing current to flow through LED 92, illuminating LED 92,which indicates to an operator of the motor that the insulationresistance has dropped to below the predetermined value, and theinsulation tester is in a fault condition.

Contact 83 opens, interrupting power to high voltage power supply 30 sothat test voltage is no longer applied across the windings and thechassis of motor 100. The opening of contact 83 also interrupts power toLED 93, causing LED 93 to shut off.

Contact 84 opens, interrupting the starter circuit of motor 100, whichprevents motor 100 from being started even if starter switch 125 issubsequently closed.

When power to comparator circuit 40 is interrupted, time delay relaycoil 60 is de-energized, and contact 61 returns to the open state shownin FIG. 1. Due to the fact that contacts 81-87 are latching contacts,they remain in their changed state even though current no longer flowsthrough latching relay coil 80. Thus, the insulation tester remains in afault condition, even if the value of the insulation resistance risesabove the predetermined value. The use of latching contacts 81-87 isadvantageous in that the operator of the motor can observe that theinsulation resistance of the windings has dropped below a predeterminedvalue, even though it may have only been for a brief period of time whenno one was observing automatic insulation tester 400.

When the operator tries to start motor 100, open contact 84 will preventmotor relay coil 120 from energizing, thus preventing motor 100 fromstarting even if the operator presses starter switch 125. The operatorthen presses reset button 94 (FIG. 9) momentarily closing reset switch91, which energizes release relay coil 90, changing the state ofcontacts 81-87 back to that shown in FIG. 1. If the insulationresistance has not risen above the predetermined value, time delay relaycoil 60 will energize again, closing contact 61, energizing latchingrelay coil 30, which again changes the state of contacts 81-87 from thatshown in FIG. 1, again opening contact 84 in the starter circuit. Thus,the operator is prevented from starting motor 100 when the insulationresistance is below the predetermined value. If, however, the insulationresistance has risen above the predetermined value, time delay relaycoil 60 does not energize, contacts 81-87 remain in the stage shown inFIG. 1, and the operator can start motor 100.

The operator starts motor 100 by closing starter switch 125, whichcauses current to flow through closed contact 84, energizing motor relaycoil 120. Energization of motor relay coil 120 causes contacts 121, 122and 123 to close, allowing current to flow from buss lines 101, 102 and103 to winding terminals 111, 112 and 113 through lines 131, 132 and133, respectively. Current is then allowed to flow from line 131 to line132 via control relay coil 70, energizing coil 70. Energization of coil70 causes contact 71-73 to open. Opening contact 71 opens the connectionbetween power input terminal 41 and terminal connection 1, interruptingpower to comparator circuit 40. Opening contact 72 interrupts power tolow voltage power supply 20. Opening contact 73 opens the connectionbetween output terminal 33 of high voltage power supply 30 and chassis110 of motor 100, breaking the electrical connection between windings114, 115, 116 and chassis 110. This prevents current from flowingthrough windings 114, 115 and 116 through high voltage power supply 30to chassis 110. If the output impedance of high voltage power supply 30is sufficiently high such that only negligible current would flowthrough its output terminals, contact 73 could be omitted. Contact 73,however, is preferably retained even if the output impedance of highvoltage power supply 30 is high, to prevent current from flowing throughhigh voltage power supply 30 in the event that a short circuit shoulddevelop therein. Fuse 35 is an additional safety feature, which can beused in addition to or instead of contact 73. Fuse 35 blows if thecurrent flowing through it rises above a predetermined value, whichvalue is greater than the current normally flowing from terminalconnection 4 to output terminal 34, and less than the value of currentwhich would flow through fuse 35 should a short develop in high voltagepower supply 30 while motor 100 is operative.

When motor 100 is shut off by opening starter switch 125, motor relaycoil 120 is de-energized, returning contacts 121, 122 and 123 to theopen state shown in FIG. 2. Control relay coil 70 then de-energizes,returning contacts 71, 72 and 73 to the state shown in FIG. 1.Comparator circuit 40 and low voltage power supply 20 again receivepower, output terminal 33 of high voltage power supply 30 is againconnected to chassis 110 and automatic insulation tester 400 is in anoperational condition. Insulation tester 400 remains in an operationalcondition until either a fault condition is detected by comparatorcircuit 40, or until motor 100 is started again.

An embodiment of the present invention which may be used with groundedelectrical equipment, such as that shown in FIG. 4, is shown in FIG. 3.

The electrical equipment shown in FIG. 4 comprises a three phase, fourwire generator 200 having a chassis 210 and windings 214, 215 and 216.Winding terminals 211, 212 and 213 of windings 214, 215 and 216,respectively, are electrically connected, Via lines 231, 232 and 233 anda circuit breaker 240 to "live" buss lines 201, 202 and 203,respectively. A neutral line 204 is electrically connected to thejunction of windings 214, 215 and 216. Also shown in FIG. 4 are thevoltage regulator 250 of generator 200, and the starter solenoid 260,start switch 261 and battery 262 of the engine (not shown) of generator200.

The circuitry shown in FIG. 3 comprises the elements shown in FIG. 1,along with additional elements which enable it to be used with groundedelectrical equipment such as that shown in FIG. 4. These additionalelements include a DC ground interrupter relay coil 320 which isconnected across the output terminals 333 and 334 of a rectifier bridge330, input terminal 331 of rectifier bridge 330 being electricallyconnected to terminal connection 1. Normally-closed contact 74,controlled by control relay coil 70, is electrically connected betweeninput terminal 332 of rectifier bridge 330 and terminal connection 2. Ifground interrupter coil 320 is an AC coil, rectifier bridge 330 could beomitted. An open-ground indicating means 340, which may be, for example,an LED or other visual indicator, is electrically connected to terminalconnection and normally closed contact 321, controlled by groundinterrupter coil 320. Contact 321 is also electrically connected toterminal connection 2. A capacitor C15 is connected in parallel withground interrupter relay coil 320.

Normally-open contact 311, controlled by time delay coil 310, iselectrically connected between latching relay coil 80 and terminalconnection 2, in parallel with contact 61.

Normally-open contact 323, controlled by ground interrupter relay coil320, is electrically connected between terminal connections 13 and 14.

For purposes of illustration, all contacts shown in FIG. 3 are shown inthe state in which they are when insulation tester 300 is installed asindicated in FIG. 4, with buss lines 201, 202 and 203 "live". Insulationtester 300, shown in FIG. 3, is installed as shown in FIG. 4, in thefollowing manner. Buss line 201 and neutral line 204 are electricallyconnected to terminal connections 1 and 2, respectively. Winding 216 iselectrically connected to terminal connection 4 via line 233. Startersolenoid 260 is electrically connected to terminal connection 5, andbattery 262 is electrically connected to terminal connection 6. Neutralline 204 is electrically connected to terminal connection 8 and voltageregulator 250 is electrically connected to terminal connection 9.Terminal connections 11 and 12 are electrically connected to neutralline 204 and line 231, respectively. Terminal connections 13 and 14 areelectrically connected to neutral line 204 and chassis 210,respectively. Terminal connection 3 is electrically connected to chassis210.

In operation, insulation tester 300 works in the following manner. Whengenerator 200 in inoperative, current flows through ground interrupterrelay coil 320, via contact 74. Ground interrupter coil 320 remainsenergized as long as contact 74 remains closed, and energization of coil320 causes contacts 321 and 322 to be closed and contact 323 to beopened, as shown in FIG. 3. Thus, when coil 320 is energized, theelectrical connection between terminal connections 13 and 14 is open,interrupting the electrical connection between neutral line 204 andchassis 210, allowing insulation tester 300 to test the insulationresistance of windings 214, 215 and 216 of generator 200 in the samemanner as insulation tester 400 tests the insulation resistance ofwindings 114, 115 and 116 of motor 100. Contact 321 allows current toflow through open-ground indicating means 340, signaling to an operatorthat neutral line 204 is disconnected from chassis 210 (that is, thatgenerator 200 is not grounded).

Should the insulation resistance of windings 214, 215 and 216 drop belowthe predetermined value, after a predetermined time period time delaycoil 60 energizes. Energization of coil 60 causes contact 61 to close,allowing latching relay coil 80 to energize and change the state ofcontacts 81-87 from that shown in FIG. 3. Contacts 81 and 83 open,interrupting power to comparator circuit 40, LED 93 and high voltagesupply 30. Contact 82 closes, allowing LED 92 to illuminate, indicatingthat insulation tester 300 has detected a fault condition. Contact 84opens, breaking the electrical contact between terminal connections 5and 6, thereby preventing starter solenoid 260 from energizing even ifswitch 261 is closed. Contact 86 opens, breaking the electrical contactbetween terminal connections 8 and 9. Insulation tester 300 is reset inthe same manner as insulation tester 400, by closing reset switch 91.

If no fault condition has been detected, or insulation tester 300 isreset and a fault condition no longer exists, the operator of thegenerator may start the generator. The operator of generator 200 startsgenerator 200 by closing start switch 261, which causes current to flowthrough starter solenoid 260, starting the engine (not shown) ofgenerator 200. Current then flows from line 231 to neutral line 204through control relay coil 70, energizing coil 70 and causing contacts71-74 to change from the state shown in FIG. 3. Contact 71 opens,interrupting input power to comparator circuit 40. Contact 72 opens,interrupting power to low voltage power supply 20. Contact 73 opens,interrupting the electrical connection between output terminal 33 ofhigh voltage power supply 30 and chassis 210 of generator 200. Contact74 opens, interrupting power to ground interrupter coil 320, whichcauses the state of contacts 321, 322 and 323 to change from the shownin FIG. 3. Contact 323 closes, establishing electrical communicationbetween neutral line 204 and chassis 210. Contact 321 opens,interrupting power to open-ground indicating means 340. Contact 322opens, interrupting power to time delay coil 310.

If all contacts change state as they should, normal operation ofgenerator 200 is unimpeded. When generator 200 is shut off, controlrelay coil 70 de-energizes, causing the state of contacts 71-74 tochange back to that shown in FIG. 3. Contact 71 closes, allowingcomparator circuit 40 to be powered. Contact 72 closes, allowing lowvoltage power supply 20 to be powered. Contact 73 closes,re-establishing electrical contact between output terminal 33 andchassis 210. Contact 74 closes, allowing ground interrupter coil 320 toenergize. Energization of ground interrupter coil 320 causes the stateof contacts 321, 322 and 323 to change back to that shown in FIG. 3.Contact 323 opens, disconnecting the electrical connection betweenneutral line 204 and chassis 210. Contact 322 closes, re-establishingelectrical contact between time delay coil 310 and terminal connections11 and 12. Contact 321 closes, causing open-ground indicating means 340to be powered. A time delay coil is sued as coil 60 in the event thatcontact 323 does not open before comparator circuit 40 is powered, andcomparator circuit 40 detects resistance below the predetermined valuebetween chassis 210 and windings 214, 215 and 216 via terminalconnections 13 and 14. Time delay relay coil 60 cannot energize untilthe low resistance is detected for a predetermined time period. The timedelay period of time delay coil 60 is set long enough to give contact323 time to open before time delay relay coil 60 can energize.Otherwise, every time generator 200 would shut off, since contacts 71-74close before ground interrupter coil 320 energizes and opens contact323, a fault condition would be detected by comparator circuit 40,causing coil 60 to energize, closing contact 61, and causing insulationtester 300 to shift to a fault condition. (Although coil 60 is shown anddescribed as a time delay relay coil in FIG. 1, it could instead be anormal relay coil, since the delay period is not essential in ungroundedsystems).

Insulation tester 300 is again in an operational condition, in which itremains until either a fault condition is detected by comparator circuit40, or until generator 200 is started again.

Should contact 74 fail to open when generator 200 is turned on, groundinterrupter coil 320 will not de-energize and the state of contacts 321,322 and 323 would not change from that shown in FIG. 3. In such a case,there would be no electrical connection between windings 214, 215, 216and chassis 210 so generator 200 would run while ungrounded, which wouldbe in violation of safety codes where grounded electrical equipment isrequired. To prevent generator 200 from running while ungrounded,contact 322, time delay coil 310 and contact 311 are provided inautomatic insulation tester 300. Time delay coil 310 would, after apredetermined time period, energize, closing contact 311. Current thenflows through latching relay coil 80, changing the state of contacts81-87 from shown in FIG. 3. Contact 86 opens, causing voltage regulator250 to stop operating, which prevents voltage from building up ingenerator 200 while the generator is ungrounded. Since contact 321remains closed, open-ground indicating means 340 remains energized, andthe operator knows that generator 200 is not operating properly becauseinsulation tester 300 has detected that there is no electricalconnection between neutral line 204 and chassis 210. He would thencontact a serviceman to correct the problem. The time delay period oftime delay coil 310 is set long enough to, under normal conditions, givecontact 74 time to open, allowing ground interrupter coil 320 toenergize and open contact 322 before time delay coil 310 energizes.Otherwise, time delay coil 310 would energize every time generator 200is started, causing latching relay coil to energize and preventinggenerator 200 from operating properly. Instead of voltage regulator 250,an air damper control circuit, for example, could be connected toterminal connections 8 and 9 such that the engine (not shown) ofgenerator 200 would shut off when contact 74 fails to open.

When buss lines 201, 202 and 203 are not always "live" and no source ofAC is constantly available, a battery would be used to power theautomatic insulation tester. In such a case, insulation tester 300 shownin FIG. 3 would not be practical, as a great deal of current isnecessary to energize ground interrupter coil 320, and a battery wouldquickly discharge if it were used to energize ground interrupter coil320. In such a case, the preferred embodiment of the presentinvention--insulation tester 500, shown in FIG. 5, would be used. Theonly difference between insulation tester 300 and insulation tester 500is that in insulation tester 500, ground interrupter coil 320 andrectifier bridge 330 are connected across terminal connections 11 and 12instead of across terminal connections 1 and 2, and contact 74 iseliminated.

FIG. 6 shows insulation tester 500 installed in the same manner asinsulation tester 300 is installed in FIG. 4, the only difference beingthat terminal connections 1 and 2 are connected to a battery 601 insteadof to buss line 201 and neutral line 204. As in insulation tester 300,contacts 321 and 322 remain closed and contact 323 remains open whengenerator 200 is inoperative, but in insulation tester 500, this is dueto ground interrupter coil 320 being unenergized, rather than energizedas in tester 300, when generator 200 is inoperative. Other than theoperation of ground interrupter coil 320, insulation tester 500 works inthe same manner as insulation tester 300. It should be noted that, whenbuss lines 201, 202, and 203 are always "live", automatic insulationtester 500 can be installed as shown in FIG. 4.

Referring now to FIG. 7, an example of a multi-input low voltage powersupply, which can be used in the insulation tester of the presentinvention, as shown. When using 110 VAC as the source of power connectedto input terminals 21 and 22, terminal 25 is connected to terminal 26,and terminal 27 is connected to terminal 28. When using 240 VACconnected to input terminals 21 and 22, terminal 26 is connected toterminal 28. When using a battery connected to input terminals 21 and22, contacts 29 are closed, allowing the DC current to bypass thetransformer T1. Regardless of whether the input voltage is 12 VDC, 24VDC, 110 VAC or 240 VAC, a stable 6 VDC is produced by low voltage powersupply 20 across its output terminals 23 and 24. The above-mentionedvoltages are exemplary only, and represent some of the more commonlyavailable supply voltages in the United States. Other voltages, ifdesired, could of course be used. In addition to the componentspreviously mentioned, also shown in FIG. 7 are a rectifier bridge REC1,capacitors C1 and C2, fuse F1, power voltage regulator PVR, resistor R1,and variable resistor R2.

FIG. 8 shows a high voltage power supply 30 which can be used in theautomatic insulation tester of the present invention. High voltage powersupply 30 receives the voltage produced by low voltage power supply 20,and converts it to a high DC voltage which is preferably greater than orequal to the rated voltage of the electrical equipment to be tested. Thevoltage produced across output terminals 33 and 34 can be, for example,500 volts. As mentioned earlier in the description of FIGS. 1 and 2,should the operator of the electrical equipment desire to insure thatthe reading displayed in display 50 is accurate, he presses test switch48 (FIG. 9) which opens test contact 45 and closes test contact 46,which applies the output of high voltage power supply 30 across testresistor 47 (FIGS. 1, 3 or 5). If high voltage power supply 30 isproducing voltage of the proper value, needle 51 (FIG. 9) of display 50will move to calibration set point 52. If not, calibration adjustingmeans 36 (FIGS. 8 and 9) is adjusted such that needle 51 moves tocalibration set point 52. Also shown in FIG. 8 are resistors R5, R6, andR7, diodes D3 and D4, transistors Q2 and Q3, capacitors C3 and C4, andtransformer T2.

Shown in FIG. 9 are resistors R9-R31, capacitors C5-C14, diodes D1 andD2, integrated circuits IC1 and IC2, variable resistors VR1-VR4,transistor TR1, Zener diodes Z1 and Z2, transformer T3, and rectifierbridge REC2. Set point adjusting means 49 (FIG. 9) allows thepredetermined value of resistance necessary to trigger a fault conditionto be adjusted. Usually, the predetermined value of resistance necessaryto trigger a fault condition would be slightly above the value at whichdamage could occur to the electrical equipment if it were to be operatedin that condition. When the automatic insulation tester is used to testinsulation resistance of a generator which needs to be always availablein the event of a power shortage, for example, a generator for ahospital, the predetermined value of insulation resistance would be sethigher, so that if a fault condition were detected by the insulationtester immediately prior to a power outage, the generator could still beused. It should be noted that, when the insulation tester of the presentinvention is being used to test electrical equipment which needs toalways be available, the insulation tester would be hooked up to theequipment such that when the insulation tester detects a faultcondition, operation of the equipment would not be impeded. In such acase, one or more of contacts 84-87 could be used to allow theinsulation tester to communicate with, for example, a computer in acontrol room to indicate when the insulation tester detects a faultcondition.

Display means 50 can comprise any suitable, preferably highly accurate,ammeter, and displays resistance instead of current. It should be notedthat visual display of resistance 50 may also comprise a digitaldisplay.

Fuse 35 is preferably placed in series between output terminal 33 ofhigh voltage power supply 30 and test input terminal 44 of comparatorcircuit 40 such that a test to determine the calibration of the displaymeans 50 will also indicate if fuse 35 is blown, by giving an infinitereading of resistance.

The ground interrupter (that is, the components of the present inventionwhich allow it to be used with grounded electrical systems) may bemanufactured separately to allow automatic insulation monitors presentlyin use to be adapted for use with grounded electrical systems.

The multiple motor tester unit 700 shown in FIG. 10 is designed to testand monitor up to nine motors with one central insulation tester unit.Unit 700 consists of one power supply and relay module 720, one remotesolid state switching control module 710, and a 1% meter indicator 50.The input power can be 12/24 VDC or 120/240 VAC. Unit 700 is equippedwith an on/off switch (not shown), test switch 48, and a reset switch94. The switching control module 710 has an auto/manual control switch711, a digital display motor indicator 712 with selector switch 713, atest LED indicator 714, and nine motor fault LED indicators 731-739. Thepower supply unit 720 requires one wire 741 connected to a common systemground 742 and one wire 743 connected to the B phase winding of eachmotor to be tested. Preferably the output 33 from the high voltage powersupply 30 is also connected to ground, with the output 44 from thecomparator circuit 40 being connected to wire 743 of each motor as willbe described. There are three sets 751, 752, 753, of contacts, for eachmotor, that can be used for motor-starter lockout and alarm circuits.The switching control module 710 requires a normally closed contact 761from each motor starter 760 when motor starter 760 is in the openposition and the motor is idle. This indicates to the switching controlmodule 710 whether the motor should be tested or skipped over (if thecontact 761 is open, module 710 skips that motor and switches to thenext motor which has a closed contact 761).

Once power is supplied to the power supply module 720 and the on/offswitch is turned on, the motor indicator 712 will display which motor isbeing tested. The yellow flashing test LED 714 will light to indicate ifthat motor is actually being tested or not light if it is being skippedover because it is on-line. By placing the auto/manual switch 711 in themanual position, the operator can choose to test any motor he wants, bypushing the motor select button 713 until the motor indicator 712 showsthe motor he desires. The motor will be continuously tested until theoperator selects another motor or that motor starts and goes on-line. Hecan now randomly select motors and test time intervals. In the auto modethe unit 700 will automatically scan through all nine motors at thepre-set time (preferably between thirty seconds and one minute permotor) interval. If at any time either in auto or manual a motor testsbelow the pre-set minimum insulation level, the corresponding red L.E.D.731-739 will light and that motor's alarm and lockout contacts 751, 752,753 will change state. The motor cannot be released from a faultcondition until it is selected again and the insulation level testsabove the set point. Then the manual reset button 94 can be pressed onthe power supply module, returning the motor to operational status. The1% meter 50 indicator will show the meg-ohm value of each motorinsulation when it is selected for testing and the yellow test L.E.D.714 is flashing. The test button 48 can be used periodically to ensurethat unit 700 is operating correctly. By first placing unit 700 in themanual mode and selecting the zero on the digital display, the testbutton 48 can then be pressed and the meter indicator should go to thecalibrate-set position. The alarm set point, test time period, calibrateset point, and set point trip time delay can all be preset duringassembly.

Details of the switching logic of the module 710 are shown in FIG. 12. Afour bit counter 900 is used to select which motor is being analyzed orselected. The counter 900 counts from zero to nine and resets to zero,thus providing selection for the nine motors of the preferred embodimentand a non-selected state. The clock input of the counter 900 isconnected to the common terminal of the auto/manual switch 711. The autoterminal of the switch 711 is connected to the output of a free runningoscillator 902. This oscillator 902 thus provides the automaticselection rate for the system. The manual terminal of the switch 711 isconnected to the output of debounce logic 904. The manual selectorbutton 713 is connected to the input of the debounce logic 904. In thismanner the operator can advance the counter 900 once for each push ofthe selector button 713.

The output of the counter 900 is provided to the motor indicator display712 and to the input of a 4 to 16 decoder 906. The output of the decoder906 associated with the binary input value goes to a low state when thatbinary value is present at the input to the decoder 906. Thus the firstten outputs of the decoder 906 cycle low as the counter 900 sequences.The one to nine outputs of the decoder 906 are connected to one input oftwo input OR gates 908. For simplification only one OR gate 908 isillustrated, the other eight being identical. The second input of the ORgate 908 is connected to one terminal of the normally closed contacts 76of the motor associated with that OR gate 908, with the other terminalof the contacts 761 being connected to ground. A resistor 910 connectedto a high logic level voltage is used to pull this second input to ahigh level, so that when the motor is activated the output of the ORgate 908 is high, indicating that this motor is not to be analyzed. Thusfor the output of the OR gate 908 to be low, both the counter 900 mustindicate that particular OR gate 908 and the motor associated with thatOR gate 908 must not be running.

The outputs of the OR gates 908 form the SELECT bus, which is providedto the power supply module 720. Additionally, a nine input AND gate 910receives the outputs of the OR gates 908 as its inputs. The output ofthe AND gate 910 is connected to the cathode of the LED 714, whose anodeis pulled to a positive level through a resistor 912. In this manner, ifany of the OR gates 908 has a low output, indicating selection of thatmotor, the test LED 714 is activated.

The fault LED's 731--739 have their anodes pulled to a high level byresistors 914 and their cathodes connected to the FAULT bus, which isconnected to the power supply module 720.

Details of the power supply module 720 are shown in FIG. 13. The SELECTbus is connected to a series of relays 950, one for each motor. Therelays 950 are connected to a positive supply level. Thus, when theoutput of the respective OR gate 908 is low, the relay 950 for thatmotor is activated. Each relay 950 includes three sets of normally opencontacts. The first set of contacts 952 are connected to the wire 743and to the comparator 40. In this manner the wires 743 of the motors areconnected to the single comparator 40 used in the preferred embodiment.As the counter 900 counts, the various motors are connected foranalysis, but the motor would be bypassed if the motor is activated doto the action of the contacts 761. The comparator 40 enables relay 60 asin the single motor version when a fault is present. The relay 60includes one normally open contact 954. The contact 954 has one terminalconnected to ground and the other to one terminal of the second contact956 of the relays 950. The second terminal of the contact 956 isconnected to the latching relay coil 958 of a latching relay associatedwith each motor. Thus both a fault must be present and the motor must beselected before that particular motor's latching relay is set.

The reset switch 94 is connected to ground and to one terminal of thethird set of contacts 960 of the relays 950. The second terminal of thecontacts 960 is connected to the release relay coils 962 of the latchingrelays. Thus the motor must be selected before the fault indication canbe cleared.

In addition to the latching relay contacts 751 provided to disable themotor and 752 and 753 provided for alarm purposes, another normally openlatching relay contact 964 is used with the fault LED's 731-739. Oneterminal of the contact 964 is connected to ground and the otherterminal to the FAULT bus. Thus, when the relay is latched, therespective LED 731-739 is connected to ground and energized.

Thus, by this multiplexing and demultiplexing only a single comparatorcircuit 40 is needed to monitor several motors.

Aside from the features described herein which enable it to be used toautomatically test several motors, unit 700 can be identical in allother respects to any of the motor testers (e.g., insulation tester 400)described herein, even though not all of the details of thoseembodiments are shown in FIGS. 10 to 13.

FIG. 14 shows multiple generator unit 800 that is designed specificallyfor four generators and operates in a manner similar to that of themultiple motor unit 700. It has circuitry to interface with groundinterrupters. Unit 800 includes a switching control module 810, faultindicating red LEDs 831-834, and other features shown in FIG. 14. In thegenerator version, selection is done in a similar fashion as in themotor case, but both outputs form the high voltage supply are providedto the generator.

Aside from the features described herein which enable it to be used toautomatically test several generators, unit 800 can be identical in allother respects to any of the generator testers (e.g., insulation tester300) described herein, even though not all of the details of thoseembodiments are shown in FIG. 14.

A remote acquisition controller (RAC) can also be provided. The purposeof the remote acquisition controller (not shown) is to serve as aninterface between the automatic testers disclosed herein and anyterminal/computer, allowing the user to access control of the switchingmodule (710, 810) from a remote terminal and to select motors fortesting and receive data back at the terminal.

This function involves an analog-to-digital conversion process and RS232hand shaking.

Information hierarchy is as follows:

1) Meg readings are timed for true and accurate measurements.

2) Analog to digital conversion is processed.

3) Data streaming to a central processing unit is formatted andtransmitted.

(a) Hexadecimal carriage control is sent

(b) Then the motor number is transmitted

(c) Followed by a three digit unit number in hexadecimal

(d) Finally a two digit hexadecimal number representing the meg reading.

Data is then captured by the receiving CPU for analysis (or justdisplayed via a terminal).

Remote selection of any motor can be done via a computer or any ASCIIterminal for continuous testing. This unit can be manually disabled forsafety at the switching control module.

Because many varying and different embodiments may be made within thescope of the inventive concept herein taught, and because manymodifications may be made in the embodiments herein detailed inaccordance with the descriptive requirement of the law, it is to beunderstood that the details herein are to be interpreted as illustrativeand not in a limiting sense.

I claim:
 1. An automatic insulation monitor for greater than twomotors/generators, each motor/generator having a chassis and at leastone winding, said automatic insulation monitor comprising:(a) powersupply means having a first and a second output terminal; (b) selectingmeans for sequentially selecting each motor/generator for monitoring fora predetermined time interval, so that all of the motors/generators aresequentially selected for monitoring; (c) switching means forelectrically connecting the first output terminal of the power supplymeans to a winding of a motor/generator selected for monitoring; (d)means for electrically connecting the second output terminal of thepower supply means to the chassis of each of the motors/generators; (e)sensing means for determining insulation resistance of themotor/generator selected for monitoring by measuring current flowingbetween the first and second output terminals of the power supply means;and (f) control means for automatically causing the automatic insulationmonitor to refrain from monitoring a motor/generator when saidmotor/generator is operative.
 2. The automatic insulation monitor ofclaim 1, wherein said control means comprises means to cause saidselecting means to skip over operative motors/generators.
 3. Theautomatic insulation monitor of claim 1, wherein said power supply meanssupplies a voltage on the order of the rated voltage of themotors/generators.
 4. The automatic insulation monitor of claim 3,wherein said power supply means supplies a voltage having a value notless than the rated voltage of the motors/generators.
 5. The automaticinsulation monitor of claim 1, further comprising:an indication meanswhich is activated when the insulation resistance of the motor/generatorselected for monitoring falls below a predetermined value; and means forcausing said indication means, once activated, to remain in an activatedstate until the insulation resistance of the motor/generator selectedfor monitoring rises above said predetermined value and said automaticinsulation monitor is manually reset.
 6. The automatic insulationmonitor of claim 5, further comprising means to prevent saidmotor/generator selected for monitoring from operating when saidindication means is in an activated state.
 7. The automatic insulationmonitor of claim 1, further comprising means for manually manipulatingthe selecting means such that the automatic insulation monitor monitorsa desired non-operative motor/generator connected thereto at the will ofan operator.
 8. The automatic insulation monitor of claim 1, wherein theselecting means includes:means for manually manipulating the selectingmeans such that the automatic insulation monitor monitors a desirednon-operative motor/generator connected thereto at the will of anoperator.
 9. The automatic insulation monitor of claim 1, furthercomprising a display means electrically connected to the sensing meansfor displaying the value of the insulation resistance of themotor/generator selected for monitoring.
 10. The automatic insulationmonitor of claim 1, further comprising motor/generator indicating meansconnected to the selecting means for indicating which motor/generatorconnected thereto is selected for monitoring.
 11. The automaticinsulation monitor of claim 1, further comprising:ground-interruptingmeans for automatically interrupting the electrical connection betweenthe winding and the chassis, when the motor/generator is selected formonitoring, of a motor/generator normally having an electricalconnection between the winding and the chassis.
 12. The automaticinsulation monitor of claim 11, further comprising means for preventingsaid motor/generator from operating in an ungrounded state.
 13. Theautomatic insulation monitor of claim 12, further comprising a displaymeans, electrically connected to the sensing means, for displaying thevalue of the insulation resistance of the motor/generator selected formonitoring.
 14. The monitor of claim 11, wherein said groundinterrupting means comprises:a contact in said electrical connection;and relay means to cause said contact to be open when saidmotor/generator is inoperative and to be closed when saidmotor/generator is operative.
 15. The monitor of claim 14, wherein saidrelay means comprises:a relay coil which energizes when saidmotor/generator is operative, causing said contact to close.
 16. Themonitor of claim 14, further comprising:means to interrupt operation ofsaid motor/generator should said relay means fail to cause said contactto be closed when said motor/generator is operative.
 17. The automaticinsulation monitor of claim 11, wherein said control means comprisesmeans to cause said selecting means to skip over operativemotors/generators.
 18. The automatic insulation monitor of claim 11,wherein the selecting means includes:means for manually manipulating theselecting means such that the automatic insulation monitor monitors adesired non-operative motor/generator connected thereto at the will ofan operator.
 19. The automatic insulation monitor of claim 11, furthercomprising motor/generator indicating means, electrically connected tothe selecting means, for indicating which motor/generator connectedthereto is selected for monitoring.
 20. The automatic insulation monitorof claim 1, further comprising motor/generator indicating meansconnected to said selecting means and to said control means forindicating that a motor/generator is being monitored.
 21. The automaticinsulation monitor of claim 11, further comprising motor/generatorindicating means connected to said selecting means and to said controlmeans for indicating that a motor/generator is being monitored.